When a steel material is to be used in any of various fields such as buildings, bridges, ships, marine structures, construction machinery, tanks, and penstocks, the steel material is made into a desired shape by welding in accordance with the shape of a steel structure for which the steel material is to be used. Recent years have seen the development of increasing large steel structures and the use of stronger and thicker steel materials at a remarkable rate.
A thick steel plate having a plate thickness of 100 mm or more is typically produced by blooming a large steel ingot produced by ingot casting and then hot rolling the obtained slab. In this ingot casting and blooming process, however, a concentrated segregation area of a hot top portion or a negative segregation area of a steel ingot bottom portion needs to be discarded. This hinders yield improvement, and leads to higher production cost and longer construction time.
On the other hand, in the case of producing a thick steel plate having a plate thickness of 100 mm or more by a process that uses a continuously-cast slab as a raw material, the aforementioned concern does not exist, but there is little working reduction to the product thickness because the continuously-cast slab is thin compared to a slab produced by ingot casting. Moreover, the general tendency to require stronger and thicker steel materials in recent years has increased the amount of alloying element added to ensure necessary properties. This causes new problems such as center porosity caused by center segregation and deterioration of inner quality due to upsizing.
To solve these problems, the following techniques have been proposed for use in a process of producing an ultra-thick steel plate from a continuously-cast slab, with the aim of compressing center porosity to improve the properties of the center segregation area in the steel plate.
For example, Non-Patent Literature (NPL) 1 describes a technique of compressing center porosity by increasing the rolling shape ratio in hot rolling of a continuously-cast slab.
JP S55-114404 A (PTL 1) and JP S61-273201 A (PTL 2) describe techniques of compressing center porosity in a continuously-cast slab by, in production of the continuously-cast slab, working the material using rolls or flat dies in a continuous casting machine.
JP 3333619 B (PTL 3) describes a technique of compressing center porosity by performing forging before hot rolling in production of a thick steel plate from a continuously-cast slab with a cumulative working reduction of 70% or less.
JP 2002-194431 A (PTL 4) describes a technique of not only eliminating center porosity but also reducing the center segregation zone to improve the resistance to temper embrittlement by, in production of an ultra-thick steel plate from a continuously-cast slab through forging and thick plate rolling with a total working reduction of 35% to 67%, holding a mid-thickness part of the raw material at a temperature of 1200° C. or higher for 20 hours or more before forging, and setting the working reduction of the forging to 16% or more.
JP 2000-263103 A (PTL 5) describes a technique of remedying center porosity and center segregation by cross-forging a continuously-cast slab and then hot rolling the slab.
JP 2006-111918 A (PTL 6) describes a production method for a thick steel plate having a tensile strength of 588 MPa or more, with center porosity being eliminated and the center segregation zone being reduced. In the production method, a continuously-cast slab is held at a temperature of 1200° C. or higher for 20 hours or more, the working reduction of forging is set to 17% or more, thick plate rolling is performed such that the total working reduction including the forging is in the range of 23% to 50%, and quenching is implemented twice after the thick plate rolling.
JP 2010-106298 A (PTL 7) describes a production method for a thick steel plate having excellent weldability and plate thickness direction ductility. In the production method, a continuously-cast slab having a specific chemical composition is reheated to at least 1100° C. and no higher than 1350° C., and is then hot worked at 1000° C. or higher with a strain rate of 0.05/s to 3/s and a cumulative working reduction of 15% or more.